


Mutually Beneficial

by Acaeria



Category: Gravity Falls
Genre: Angst, Demonic Possession, Dissociative Identity Disorder, Gen, Headspace, Memory Erasing Gun, Multiple Personalities, Other Additional Tags to Be Added, Pain, Self-Harm, Self-Hatred, and knowing bill there'll probably be some, later on in the story, like a lot of it
Language: English
Status: In-Progress
Published: 2015-10-04
Updated: 2015-10-08
Packaged: 2018-04-24 19:30:27
Rating: Not Rated
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 2
Words: 4,063
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/4932478
Author URL: https://archiveofourown.org/users/Acaeria/pseuds/Acaeria
Summary: <blockquote class="userstuff">
              <p>If Bill Cipher can possess Dipper Pines, his goal will be in his grasp. The only thing is, Dipper doesn't back down easily. </p>
<p>The result leaves both of them in a place they'd rather not be.</p>
            </blockquote>





	1. Fill My Head

**Author's Note:**

> ...Honestly, I don't even know what I was planning with this one. It's going to be multi-chapter, but more of a series of interconnected one-shots with common themes but no overarching plots. Or maybe I'll think of one and there will be.
> 
> As you can tell, I have no idea what I'm doing, but I hope you enjoy this, anyhow.

He didn’t know how it had come to this.

They’d erected the shield around the mystery shack; Bill couldn’t get them while they were inside. Maybe he was always watching, but they were protected from him. Yet, he was inside; not just inside the shack, but inside Dipper’s head.

And his head was pounding.

No, it was more than that, a persistent ache, his brain splitting in two. All he wanted was to dig his fingers into his skull, because that would hurt less. Gasping in pain, he crumpled to the floor, biting back a scream.

The bathroom door opened. “Hey, Dipper, have you seen- what’s wrong?” Mabel was by his side, her hand on his shoulder, shaking him. He wanted to push her away, tell her to stop it- both to protect her and because the movement was sending bullets of pain into his brain. “Grunkle Stan! Grunkle Stan! Something’s wrong with Dipper!”

Dipper groaned, and then suddenly, the world was no more.

Instead, he was trapped in darkness, alone except for Bill, who was floating before him, the two of them separated by a thin, translucent membrane.

“Pine Tree!” Bill greeted. “How nice of you to join me. Why don’t you help me break this thing-” he tapped on the barrier, causing it to ripple- “So we can get out of this hell?”  
“How are you even here?” Dipper asked instead, ignoring the demon’s request. “We Bill-proofed the Shack, I mean, this isn’t even-”  
“Possible? Listen, Kid, nothing keeps me out for long. Not if there’s something I really want. And you know what I want? Your meat sack, so I can nip downstairs for that Rift. Now, the barrier?”

Dipper took a step back. “What happens when that barrier breaks?” he called out to Bill.

“Your body becomes mine,” Bill said nonchalantly, “And you die.”

“Yeah, I don’t think so,” Dipper said. Almost unconsciously, he drew up the anger he was feeling, the pain from his body, and flung it forward, strengthening the barrier, which took on an angry shade of red. Bill laughed, body glowing red in rage.

“You’re making a mistake, Pine Tree!” he called. “You can’t hold me back forever, and when I get my hands on you, I’ll make you s ͟u͝ f f̨ ͢e r̨.”

The darkness receded, and Dipper felt as if he were speeding away from Bill, until he jerked awake in his own body. He felt the couch beneath him, heard voices around him, but he struggled to figure out what was going on. He blinked open his eyes and groaned, and the voices came to a stop.

“Dipper?” Mabel said quietly, at the same time as Grunkle stan said,

“Kid?”

“Yeah?” he croaked, turning his head to look at them and blinking several times until they weren’t so blurry. Ford was there, too, suspicion in his face.

“Dipper, do you know what’s happening to you?” Ford asked. Dipper shrugged.

“Kind’ve?” he mumbled, hissing in pain.

“Kind of?” Ford pressed.

“It’s Bill,” he said. “He’s in my head. You need-” He winced, body contorting as a wave of pain flooded through him- “To get him out. Whatever it takes, _please_. I can’t hold him back for long.”

“But you _can_ hold him back?” Ford asked. Dipper nodded.

“Yeah.”

“Then try your hardest to do so. Mabel, come with me. We’ll find something to help your brother.”

“Are you gonna be okay, Dipper?” Mabel asked hesitantly as she went to follow Ford. Her voice was quieter than Dipper had ever heard it.

He gave a strained smile. “I’ll be fine, Mabel. Go.” She nodded, grabbing his hand and squeezing it before letting go and chasing after their great-uncle.

It was getting really hard to breathe. Dipper found himself taking deep, gasping breaths, the agony in his head spilling throughout every cell in his body.   
“Is she gone?” he asked Stan when he had enough breath to spare. His Grunkle nodded.

“Yeah, she’s gone.”

“Good.” With that, Dipper rolled himself over on the couch, stifling a whimper at the pain, grabbing a pillow and screaming into it.

The moment he ran out of breath, he was back in the darkness.

The barrier was in a worse shape than it had been. Both the translucent barrier and the red barrier had cracks in them, the former even having gaping holes at some points. Bill was still glowing red with anger, larger than Dipper had ever seen him before.

“Well, well, well, if it isn’t Pine Tree! Come on, Kid, just take down the barriers. It’ll be better if you do.”

“I’m not falling for that,” Dipper retorted. wondering just how he was going to patch up the holes.

“It’s not a _trick_ , kid! Look, how much _pain_ are you in? A lot, I’m guessing; your puny human body can’t handle it. If you let me in, it’ll all go away. Don’t you want that?”

Dipper hesitated. Put like that, it was tempting, but then he remembered what Bill had said the consequences of breaking the barrier would be: he would die. And he couldn’t die, not yet: he wasn’t even thirteen years old!

So, there was only one option, and that was to build up the barrier some more. He couldn’t fix them, but he could replace them, building a new wall for every one Bill broke through. With new determination, he pulled the emotions from his chest- fear and hatred, everything the demon made him feel- and flung it forward, creating a new barrier, shining orange.

Bill screamed and Dipper woke up.

His entire body ached and he felt cold, drenched in sweat. His breath was coming quick and shallow, and he felt shaky and weak as he pushed himself up to a sitting position.

Stan jerked to attention and held out a glass of water. “Thirsty?” he asked. Dipper took it gratefully.

“Thanks,” he said, before downing the entire thing. Stan watched him blankly, not saying a word as he took the empty glass and put it down on the wooden floor. They sat there in silence, with only the drone of the TV in the background. Neither of them were watching it, Dipper knew, just staring at the screen blankly.

After a while, the dull, resounding pain in his head began to spike again. He groaned, sinking down in his seat a bit. _I don’t know how much more I can take_ , he thought, and it startled him. He felt weak, like his entire body was so close to just collapsing and falling apart; how much longer could he keep this up?

With that thought, he was back in the darkness, and the first two barriers were gone, their fragments scattered with the demon’s rage. The final barrier, the orange one, was strewn with cracks, like glass soon to shatter.

Bitterly, choking back a sob at the absolute agony, Dipper wove a new barrier, this one from all his hopelessness and emptiness and dread. It came into being, dark and light blue intermingling on a pane like ice, constructed between them in the blink of an eye.

The orange barrier shattered and Dipper was awake once more.

Stan was staring at him. Dipper gasped for air and buried his head in his knees, choking on tears.

“Kid?” his Grunkle said softly. Dipper couldn’t reply.

_It hurts so much_ , he wailed inwardly. _It hurts so much it needs to stop I can’t do this I can’t I can’t_ -

Not even two minutes and it was dark again. Dipper curled up into a ball in the darkness, shaking even within his own head.

“Giving up?” Bill asked him pointedly. Dipper didn’t respond, just curling further into himself.

It would be nice, he thought numbly, to give up. He was so tired, in so much pain that nothing mattered. And what would happen if he gave up? He would die. Bill would be free to end the world. Everyone would die.

_Mabel would die_.

The thought filled him with horror and he found himself sitting up, staring up at the shattered blue barrier. There was barely anything left of it; it was a surprise that it was holding Bill back at all. He knew without a doubt there was only one more barrier he could make; after that, his energy would be spent. This was his last chance, his last barrier, and he knew he had to make it count.

Unlike the painful, explosive forces of the previous barriers coming into place, this one simply appeared, accompanied by feelings of warmth and comfort. It was made entirely of love, or something akin to it; summer afternoons and the smell of baking cookies and the taste of sweets and his sister’s laugh. It was made of pure sweetness, pure sunshine, glowing red and gold.

He could only hope it would hold.

He heard Bill laughing, and fell to the floor, detached from himself as his eyes drifted closed. _I’m sorry, Mabel_ , he thought, falling into an even deeper darkness, never to resurface.

Except, he did.

* * *

 

Ford and Mabel were back, and they were talking to Stan. Their voices weren’t far away at all, he could hear them perfectly well, but he couldn’t understand a word they were saying. Everything felt strange, and his body didn’t feel like his own (but that was fine, because it didn’t hurt anymore, either). Nothing was the colour it should be, and shapes were floating in front of his vision.

He tried to focus on what his grunkles and sister were saying, and was successful a moment later, comprehending what Stan was saying.

“It could kill him!”

“Yes, but it’s the only thing that could save him,” Ford was saying. He held something in his hands that was extremely similar to a memory gun.

“What’s going on?” he called, hearing the words moments after he felt his mouth move (or was it the other way round?).

“Dipper!” Mabel cried, running over to him. She grasped his hands, and Dipper waited for the sensation to join the sight of it, but nothing ever came. “Dipper? Are you… okay?”

“Not really,” Dipper admitted. His gaze drifted over to Ford. “Did you find something?” he asked.

“Yes,” Ford said, at the same time as Stan snapped,

“No.”

Ford sighed. “I found something, but if it goes wrong, you could die,” he told Dipper.

“I’ll die anyway,” Dipper pointed out, and he could’ve sworn he felt Stan’s breath catch in his throat and the gun slip from Ford’s hands and mabel’s heart stop, for only a second. What was happening to him? He wanted to grab his head and cry and scream, but he couldn’t even move.

What a pathetic way to die.

“Are you sure about this, Dipper?” Stan asked, a pleading edge to his voice. His eyes seemed to be begging dipper to reconsider. Unfortunately, Dipper didn’t have that option anymore. He felt cold, numb, and soon, he knew, his limbs wouldn’t even be his own.

“Hit me,” Dipper said, the words sounding odd and slurred to his ears, which heard them before his mouth even moved.

Ford aimed the gun at him and Mabel ducked out the way and Stan covered his eyes and suddenly everything was bright white and burning.

Then, there was nothing.

* * *

 

The next time Dipper was aware of anything he was back in his own head and the barriers in the darkness were gone. Bill was hovering before him, looking incredibly despondent- or as depondent as a triangle could look, anyway.

“What’s going on?” he asked. Bill gave him a dead-eyed look and sighed.

“It’s over.”

“What?”

“I was stupid. I put all my power into breaking into your body.”

“What does that mean?”

“It means that this is all that’s left of me, Pine Tree. As soon as I leave your head, I’ll die.” Dipper snorted.

“Get on with it, then,” he said. Bill glared at him.

“It won’t be all sunshine and happiness for you when I leave either, Kid!” he crowed, and Dipper started.

“What do you _mean_?” he asked.

“You’ll have no memory. You’ll have lost your ability to move. All you’ll be able to do is look and breathe- and what kind of life is that?”

“You’re lying.” Dipper’s heart was beating faster than it ever had. Bill had to be lying, he _had_ to be…

“Sorry, Kid,” Bill said. Then, “I still have a little power left. Why don’t we make a deal?”

“I’m never making any deals with _you_.”

“But this one’s mutually beneficial. Hey, kid, listen to me.” He disappeared and reappeared in Dipper’s face, causing him to scramble backwards. “You let me live in your head from now on, and you’ll keep all the things that make you a functioning human being!”

“Why should I trust you?” Dipper asked suspiciously.

“At this point, I have no reason to lie. And I am the only chance you have left. So, what do you say- do we have a deal?”

Hesitantly, haltingly, and full of self-hatred, Dipper reached out and took the demon’s hand. “Deal.”

* * *

 

Dipper opened his eyes.

Mabel, Stan and Ford were all staring at him; Mabel from behind the couch, Stan from between his fingers and Ford from behind the gun. The end of it was still smoking.

“Did it work?” Mabel asked.

“Yeah,” Dipper said, and suddenly, the true reality of the lack of pain he was experiencing dawned upon him and he laughed. “Yeah, it worked!”

 

 


	2. Quite the Problem

**Summary for the Chapter:**

> In which Mabel discovers that there's a demon in her brother's head, and embarks on a quest to get him out.

**Notes for the Chapter:**

> And chapter two! I now have some idea of where I'm going with this fic, though I don't know when I'll update and what the next chapter'll contain. Anyway, I hope you enjoy this.

Dipper had been quiet recently.

Mabel couldn’t deny that she was concerned, but she did her best to hide it; worrying was her brother’s thing, not hers. She’d wait for the issue to resolve itself, and if it didn’t, then she’d solve it herself. It’s not like she didn’t know why he was doing it, after all; he was likely still recovering from Bill.

At least the demon was dead now, and Dipper could finally get over it. Then the two of them would be entirely back to normal.

Or so she thought until she came downstairs one morning to find Dipper sat at the kitchen table, holding a journal and writing down elaborate codes at the speed of light. Now, this usually wouldn’t be so strange, except that his work was dotted with the familiar triangle symbol.

“Uh, Dipdop? What’cha doing?” she asked cautiously, peering over his shoulder from several steps away. Slowly, with unnaturally jerky movements, he turned his head and grinned at her. The grin sent shivers down her spine; it seemed more fitting on another face, one with glowing yellow eyes and an unnaturally pitched voice…

“Try again, Shooting Star,” said a voice that was both her brother’s and not, and Mabel felt like she’d been dunked into a bath of cold water.

“ _Bipper_!” she hissed, taking a step backwards, towards the door. “How are you here? You’re supposed to be dead!” Then, “Leave my brother alone!”

He laughed, head tipping back with mirth. “Oh, you mean he didn’t tell you? Man, this is great!” Mabel glared suspiciously at him.

“What do you mean?” she asked hesitantly.

“Oh, you remember when Sixer tried to erase me right out of Pine Tree’s mind? You remember that, right? Because, well, it didn’t work! Or, it did: it diminished all but a deal’s worth of power from me, meaning that I’d die if I left his head, and it erased your brother’s very _mind_.” Mabel stared at him in confusion, and Bipper smirked. “But, like I said, I still had one deal left, right? So we made a deal: he’d keep his mind and I’d stay in his head.”

“Well, get out of it!” she snapped. “Leave him alone, haven’t you bothered him enough already? You _jerk_.”

“Sorry, Shooting Star, no can do. You see, I’m a permanent part of Pine Tree now. I’m ingrained in his brain; it’s my mind as much as his. I’m gonna be here until the day he dies.”

Mabel gritted her teeth. Silence stretched in the kitchen, the ticking of the clock the only sound she could hear. Tears stung her eyes, and anger filled her; she wanted to punch something, throw a tantrum, force the demon out of her brother’s psyche.

She quenched the emotions down, took several deep, shaking breaths, and spun on one foot to leave the room. “Where are you going?” Bipper called after her, but she didn’t reply, instead heading upstairs to their bedroom and grabbing the journals from the shelf above Dipper’s bed. She opened them and flicked through, starting at the third and working her way back.

She finally found what she was looking for towards the end of the first book, and by this time, Bipper had followed her upstairs and was perched on the end of her own bed, watching her curiously. She bookmarked the page, grabbed her grappling gun, and stormed downstairs, out of the shack.

Bipper followed her as she made a beeline for the woods. “Hey, Shooting Star?” he called, running to catch up to her. “What’s this about?” he asked as he slowed to match her pace. She shook her head, opening the book to check the map. Bipper peered over her shoulder and let out a sound of understanding.

“Ohh, an oracle! Is that what we’re going to see? It’s pointless, you know, but if it’ll make you feel better…” He looked at her and then gave a sigh. “Not talking to me, huh? Is this some sort of grudge thing? I’d apologise if you want? Do you want?”

Mabel groaned. “Just leave me alone!” she snapped, turning on him as she slammed the book closed. “I don’t want you here! That’s why I’m going to the oracle in the first place, to get away from you!”

“Wow, was that anger? I don’t think I’ve ever seen you so angry, Shooting Star. Should I be honoured?” Mabel sighed in frustration, turned and continued walking, Bipper walking in time with her, matching her stride for stride. “Eh, I don’t care if you want me here, I’m coming anyway.”

The rest of the journey was mostly silent, with Bill calling out to her when she went the wrong direction or was about to fall into a hole or stream or trip over a root or something. At first, Mabel purposely ignored him, but after the first few times of getting hopelessly lost or falling in the mud, she decided it would be for the best to follow his instructions.

Finally, they came to a small, marshy grove covered with weeds and tough roots, a half-sunken wood-and-stone path leading to the door of a moss-coated stone cottage. Grimacing at the rotten marsh-smell, Mabel made her way across the bridge, leaping across and dodging the particularly bad regions of the path. Bipper followed her, stepping exactly where she stepped with such accuracy and precision it made her anxious.

Upon reaching the door, she reached up and took the door knocker, which was rusted bronze and in the shape of a woman with snake hair. She didn’t look very friendly, with her mouth twisted open in an eternal scream, showing wicked-sharp teeth and a pointed tongue.

The sound of the knock rang through the still air, and Mabel heard movement from inside, wood scraping against stone. There were footsteps, and then the door opened, to reveal… A girl her own age, wrapped in a shawl and wearing a ragged dress, eyes hidden in shadow and a mouth stretched in a hesitant but then welcoming smile.

“Oh, welcome!” she greeted warmly. “Come inside, come in!” She stepped back, and Mabel stepped inside, looking around the small room inside. The walls were bare brick, and the floor bare wood, and the furniture was plain and splinter-y. The air was cold and smelled of damp, and Mabel’s nose screwed up in disgust. This didn’t seem a very nice place to live.

The door closed behind them, and the girl showed them to the couch before perching on a wooden stool before them.

“My name’s Pythia, it’s nice to meet you,” the girl said warmly. “But, you have come here for a reason? Can you tell me what it is?”

And so Mabel explained Dipper’s problem to her, ignoring both Bipper and his occasional additions and butt-ins to the story. “So, I decided to come here, to find a way to fix this,” she finished with a deep breath. Pythia frowned.

“That’s… quite a predicament. I can… consult the oracle for you, but I cannot help the answer you’ll find.” Mabel frowned.

“But aren’t you the oracle?” she asked, and Pythia nodded.

“Yes,” she said, pulling the shawl from around her head to reveal limp black hair and glassy, pale-green eyes, devoid of pupil or iris. She opened her mouth, and from it green smoke spilled, wreathing around Mabel and Bipper.

Soon Mabel could not see anything for the smoke and she really wished her brother were here to see this with her.

Then, she heard words whispering in her ears, and images played in the smoke before her: her and Dipper, running through the woods; a pile of discarded, torn-apart, and unravelled sweaters; a broken mirror, glass falling to the floor, spattered with blood; a girl with dark hair and bright eyes and an excited grin; a bus, making its way through a winding road lined with pine trees; Dipper, tears rolling down his cheeks; Dipper, wreathed in flame.

The words in her ears said: “ _There is no solution to your problem. It’s not something that can be fixed. It is something that can be accepted, however, and that is what you must do: accept. Learn. Grow._ ”

Then Mabel was blinking and the smoke was receding and Pythia’s hood was back on and she realised that she was gripping Bipper’s hand, and he looked as shaky and pale as she felt.

“Did you find your answers?” Pythia asked. Mabel forced her trembling lips shut and nodded. “Then you must go.”

Bipper was up on his feet in seconds and racing to the door, flinging it open and racing into the cold. Mabel stood more slowly, her legs feeling like jelly. Even now, she could hear her heart hammering in her head.

“Um, Pythia?” she asked, half way across the room. “Would you mind if I came back?” Pythia frowned.

“Back?” she asked. “What for?”

“Well, I mean, this place stinks. I could help you decorate! Make it warmer, more comfortable.” A look of shock appeared on the girl’s face. “I mean, if you don’t want to, I won’t-”

“No!” Pythia cried, before clearing her throat and saying more calmly, “No, it’s okay. I’d like it if you came back.” Mabel grinned.

“Great! I’ll be back soon, then. See you!”  
“See you,” she heard Pythia echo as she closed the door behind her, racing out across the bridge and to the edge of the forest, where Bipper was waiting. He was leaning his head against a tree, pale and covered in cold sweat.

“Bill?” she asked carefully, “Are you okay?”

“Puny humans,” Bipper muttered painfully, “with your puny biology. I feel- what do you call it?- sick. Very sick.”

She reached out and grabbed his arm, offering him a comforting smile (or as comforting a smile as she could muster, considering this was the demon that had tried to kill her brother). “Come on,” she said, “Let’s go home. You can lie down and sleep it off.” He paused, grimacing, before nodding.

“Okay,” he said, and the two of them set back down the path toward home, encased in a semi-comfortable silence. The oracle’s words still echoed in her head: _It is something that can be accepted, however, and that is what you must do: accept. Learn. Grow_. Now, Mabel liked to think of herself as a very open-minded person, but she didn’t know if she could put up with the fact that a demon was now stuck in her brother’s head, permanently. After all, who knew what kind of trouble he could cause?

 _He hasn’t been in any way threatening_ , a treacherous part of her reminded herself. _In fact, he’s been civil and helpful, if unstable. Maybe he won’t be so bad?_

Still, she’d keep an eye on him, for her brother’s sake as much as hers. 


End file.
